1. Field of the Invention
In general, this invention relates to an integrated-circuit operational amplifier. More particularly, it relates to such an amplifier having an open-loop gain versus frequency characteristic which provides stable and accurate closed-loop operation.
2. Description of the Prior Art
An operational amplifier is a specific type of circuit generally classified as a linear or analog circuit. The general function of an operational amplifier is to produce an output signal having a magnitude that is a linear function of the difference between the magnitudes of two input signals, one of which is applied to an inverting input node and the other of which is applied to a non-inverting input node. It is possible to employ a so-called single-ended input stage within an operational amplifier, in which case one of the input nodes is connected to a fixed reference such as ground or 0 volts. In integrated-circuit operational amplifiers, it is generally desirable to employ a differential amplifier as an input stage.
An operational amplifier has utility in numerous circuit arrangements. For example, U.S. Pat. No. 4,868,482, which is assigned to the assignee of the present application, discloses various integrated-circuit operational amplifiers, each of which incorporates a differential amplifier as an input stage. The referenced patent further describes and shows in its FIG. 5 an advantageous arrangement of an operational amplifier which is incorporated as part of an overall circuit for providing a gate-to-source biasing voltage for a field effect transistor (FET) such that the drain-to-source resistance of the FET defines a precision resistor. Additional FETs biased by the same biasing voltage are provided to define additional precision resistors.
As another example of an operational-amplifier application, an operational amplifier can be used within an integrated circuit that provides digital-to-analog conversion, wherein the operational amplifier supplies a precise analog reference signal, either in the form of a reference voltage or reference current.
In each of a variety of overall circuit arrangements in which an operational amplifier can be incorporated, it is highly desirable for the operational amplifier to exhibit certain characteristics. One highly desirable characteristic is high open-loop gain at low frequencies. Another highly desirable characteristic is less than unity open-loop gain at high frequencies at which phase shifts cause positive feedback.
An example that underscores the desirability of high open-loop gain involves an operational amplifier within a voltage-follower circuit arrangement. In such an arrangement, a feedback path is provided such that the output signal of the operational amplifier is applied to an inverting input node of the operational amplifier, and an input signal is applied to a non-inverting input node of the operational amplifier. The following equation gives the magnitude of the output signal (e.sub.o) as a function of the magnitude of the input signal (e.sub.i) and the open-loop gain (G) of the operational amplifier: EQU e.sub.o =(e.sub.i)(G)/(G+1)
As indicated by the foregoing equation, the magnitude of e.sub.o asymptotically approaches e.sub.i as G increases. Generally, the open-loop gain G is subject to substantial variation, because of temperature changes, power supply variations, device tolerances, etc. However, the percentage change in the magnitude of e.sub.o is substantially less than the percentage change in open-loop gain G, and the higher the minimum value of G, the less percentage change in the magnitude of e.sub.o will result from changes in the value of G to its maximum.
As indicated above, another highly desirable characteristic of an operational amplifier relates to its gain and phase shift as a function of frequency. If, in the above-described example of an operational amplifier in a voltage follower circuit, the gain of the operational amplifier were to be greater than unity at a frequency at which the closed-loop phase shift were 360.degree., the circuit would be unstable, i.e., it would oscillate. To prevent such oscillations, stability compensation is desirably provided for an operational amplifier.
Stability compensation for integrated-circuit operational amplifiers is normally accomplished by incorporating compensation circuitry that is said to create a dominant pole at a particular frequency. That is, the compensation circuitry operates somewhat like a low-pass filter which has little or no effect on open-loop gain at low frequencies and which reduces or rolls off the open-loop gain at frequencies above the dominant pole. The rate of roll-off of gain caused by a single pole is typically expressed as 20 db per decade; this means that the gain decreases by a factor of 10 as frequency increases by a factor of 10. A phase shift is introduced by each such pole; this phase shift asymptotically approaches 90.degree..
One prior art approach for providing stability compensation for an integrated-circuit operational amplifier is disclosed in U.S. Pat. No. 4,714,895. This approach relies upon switched-capacitor techniques by which a resistive load can be defined by periodically switching a capacitor into and out of connection with a circuit. An operational amplifier in accord with this approach involves a single-stage differential amplifier, 4 capacitors, and 4 clocked single-pole, double-throw switches that cooperate with the capacitors to define a common mode feedback circuit. This approach involves complexities including the need to provide clocking signals and to provide switching circuitry to implement the 4 switches. Further, as described in this patent at column 5, lines 4, et seq., the common-mode feedback circuit constitutes an internal load on the differential amplifier. As elsewhere indicated in this patent (column 2, lines 5, et seq.), the gain of a differential amplifier is affected by its loading. This loading tends to counteract a goal of providing high open-loop gain which is sought to be obtained in accord with this patent through use of complementary symmetry metal oxide semiconductor (CMOS) transistors, and the use of cascode circuits as in the differential amplifier.
Another prior art approach does not require the complexities involved with switched capacitor techniques. Instead, the operational amplifier has 2 stages, the first or input stage being a differential amplifier, the second stage being a single-ended stage comprising a signal-inverting transistor to which a compensation capacitor is connected to produce a so-called Miller effect. As to the Miller effect, this means that the second, single-ended stage capacitively loads the differential amplifier stage by an amount that is larger than the capacitance of the capacitor itself. In this regard, the capacitive loading on the input stage can be expressed as a ratio, the numerator being the magnitude of the current flowing between the two stages, and the denominator being the time rate of change of the voltage at the node at which the two stages are interconnected. The Miller effect is present when the numerator increases as a result of the opposite ends of the capacitor having oppositely-changing potentials.
For applications in which the open-loop gain needs to be very high, e.g., in excess of 10,000 (80 db), to maintain closed loop accuracy at d.c., an operational amplifier in accord with the above-described conventional approach requires an undesirably large compensation capacitor. This drawback is particularly acute in integrated circuits in which the transistors are metal oxide semiconductor (MOS) field effect transistors. In an operational amplifier in which an input-stage MOSFET differential amplifier is employed to drive a second-stage, single-ended, inverting MOSFET, it is extremely difficult to achieve stability compensation through use of the conventional approach of incorporating a capacitor connected between the drain and the gate of second-stage MOSFET. This conventional approach is particularly undesirable in an application in which the second-stage MOSFET is connected to drive current through a resistor instead of a high-resistance circuit such as an FET constant current source. Typical such applications involve precision current sources, and voltage ramp generators, particularly where small size is required. While driving current through a resistor, the second-stage MOSFET has significantly reduced drain impedance and resulting lower gain. Such lower gain undesirably reduces the intended Miller effect. For these reasons, the conventional approach is undesirably subject to instability where the load presented to the second-stage MOSFET affects its gain.